1. Field of the Invention
This invention relates to an ignition timing control system for an internal combustion engine having means for prevention of knocking irrespective of the octane value of fuel used in the engine.
2. Prior Art
Generally, if a fuel having a low octane value is used in an internal combustion engine which is designed to operate on a fuel having a high octane value, knocking can occur with considerable frequency. To overcome this problem, an ignition timing control system for an internal combustion engine has been proposed by the assignee of the present application by Japanese Provisional Patent Publication (Kokai) No. 4-101067, which comprises means for detecting occurrence of knocking, and means for retarding the ignition timing when occurrence of knocking is detected
In the proposed control system, a plurality of control zones different in the ignition timing control characteristic according to the octane value of a fuel used are provided, by which the amount of advance of ignition timing is restricted under predetermined conditions, and the ignition timing is separately controlled in each of the control zones selected by an ignition timing retarding amount determined according to the degree of occurrence of knocking. When load on the engine exceeds a predetermined value while knocking does not occur, the use of the control zones is canceled by making a changeover from a control zone which is currently applied to a control zone suitable for a fuel having a higher octane value, i.e. to a control zone where the ignition timing can be set to a more advanced value, to thereby enable to control the ignition timing in a manner suitable to the octane value of the fuel used without causing hunting in the controlled ignition timing.
However, in the above proposed system, whether or not the use of the control zones should be canceled is made only based upon the load condition of the engine. As a result, when the coolant temperature of the engine is high or when it is low, or when the engine rotational speed is low, or when the intake air temperature of the engine is high, if the control zones are used for knocking control, no changeover of the control zone will be effected, once a control zone has been selected to correct the ignition timing in the retarding direction, which results in degraded driveability. More specifically, when the engine has not been warmed up, that is, when the engine coolant temperature is low, unstable combustion occurs. Therefore, if the ignition timing is corrected toward a retarded side by the use of the control zones on such an occasion, it will result in a drop in the engine output, promoting the instability of combustion. Particularly, when the engine rotational speed is low, the combustion can be remarkably unstable, largely degrading the driveability.
Further, when the engine intake air temperature is high or when the engine coolant temperature is high, the engine cannot fully afford to avoid knocking so that knocking can occur even when a fuel having a high octane value is used. In addition, occurrence of knocking can cause erroneous determination of the control zone to be selected. As a result, the ignition timing is restricted to a value on the retarded side with respect to a desired ignition timing, so that the engine output cannot be enhanced to a desired level even when a high octane value fuel is used.